1. Field of the Invention
The field of this invention is directed to the water desalination apparatus which extracts fresh water from sea water or from other types of brackish water.
2. Description of Prior Art
Desalination is a process of converting sea water, which contains 35,000 parts per million of salt, or brackish water, containing 5,000 to 10,000 parts per million of salt, to fresh water suitable for human consumption, household and industrial requirements. The salt in drinking water should not exceed 500 parts per million. Where natural supplies of this quality of water are insufficient, desalination is an alternative to transporting fresh water over long distances by pipeline or tank vehicle.
Many types of evaporators have been used to produce fresh water by distillation. But in the past, the prior art types of evaporators have proved costly and troublesome when used continuously on the large scale which is required. At temperatures over 160 degrees Fahrenheit, sea water and many brackish waters deposit scale (incrustation of insoluble chemical compounds, especially calcium and magnesium salts) which interfere with the operation of the evaporator.
Water normally boils at 212 degrees Fahrenheit, but by reducing the pressure, it will boil at a lower temperature. Hot sea water is exceedingly corrosive. Most prior art methods of removing fresh water from sea water have been most inefficient. Modern desalination processes try to avoid the inefficient operations of the prior art by using forms of low temperature desalination for removing fresh water from sea water.
One modified form of low temperature desalination is vapor compression desalination. Like any other gas, the temperature of steam can be raised by compression. If the compressed steam is then condensed at the higher pressure, its latent heat can be returned to the boiling liquid it came from. This principle is used in the vapor compression desalinator, where the energy is supplied not as heat but by mechanical work. In this process hot sea water is introduced into an evaporator and the steam produced is drawn off and compressed, which raises its temperature as well as its pressure. This steam is then fed into another part of the evaporator where it condenses into fresh water. As it condenses, it heats up the sea water providing more evaporation. The condensed fresh water is quite warm and on its way to storage it is passed through a heat exchanger where the heat is extracted and used to heat up the incoming sea water supply.
The first vapor compression evaporators using mechanical compressors were built in Europe about one hundred years ago. Since that time, thousands of mechanical vapor compression evaporators have been put into service around the world for desalting sea water and brackish water. Several hundred evaporators have been installed in the chemical processing industries to concentrate solutions ranging from inorganic chemicals to temperature sensitive food products. The primary advantage of mechanical vapor compression evaporation over conventional arrangements is far lower energy consumption. The rapidly rising prices of oil and other energy sources in the past have been an increasing incentive to investigate vapor compression evaporation of a means of reducing energy usage and the energy contribution to product costs.